This invention relates to the design, construction and material of headers, and more particularly to the design, construction and material of a header used to manifold the solar absorption tubes of a solar absorption panel in a molten nitrate salt solar central receiver.
Previously employed molten nitrate salt solar receiver panels have used nozzles or tubes that are corner or socket welded directly to a header wall. This type of construction subjects the weld to very high, unacceptable thermal strains which are located near the edge of the opening in the header. Attempts to avoid placing the weld near the highest thermal strain region have involved using a machined nozzle which is insert welded into the header wall to locate the weld away from the highest thermal strain region. Also, tube nozzles that are located at low header flow regions required thermal sleeves or other thermal transient protection approaches to mitigate the severe thermal strains at the tube nozzle-to-header wall junction. These thermal strains are caused by the rapid temperature changes in the molten salt fluid flowing through the header due to cloud passage over a solar plant""s heliostat field. Providing this thermal protection has resulted in significant cost increases and complexity in fabrication and inspection of header assemblies used with molten nitrate salt solar receiver panels. Without this additional thermal protection, unacceptably short operational lifetimes will be experienced with header assemblies used in molten nitrate salt solar receiver panels.
Previous approaches to header design have also been predicated upon the use of either 316 or 304 stainless steel or Incoloy 800 for the solar absorption panel header. Although these materials have good molten nitrate salt corrosion resistance up to about 600xc2x0 C., their high thermal expansion coefficient, relatively low strength and moderate thermal strain fatigue resistance results in the need to provide thermal protection at the important nozzle-to-header region to meet the numerous, severe thermal transient cycles experienced by the header assembly during cloud cover motion over a heliostat field.
It is therefore a principal object of the present invention to provide a header assembly suited for use in a molten nitrate salt solar receiver panel which even more effectively deals with the thermal strains experienced at the header-to-nozzle junction area without the use of complex and expensive thermal protection devices.
It is another object of the present invention to provide a header assembly for use in a molten nitrate salt solar receiver panel which includes a thin wall small diameter header whose nozzles better thermally match the thin receiver tubes attached to the header at these nozzles, and where the header has a low thermal expansion coefficient which yields lower thermal strains in the tube nozzle-to-header region than previously developed header assemblies.
It is still another object of the present invention to provide a header assembly having a construction material that is readily weldable, formable and ASME (American Society of Mechanical Engineers), Boiler and Pressure Vessel Code (ASME Code) approved.
The above and other objects are provided by a thin wall small diameter header assembly. The header assembly incorporates extruded or machined insert-welded nozzles constructed from a nickel-based superalloy for the distribution and collection of fluid flow to and from the solar absorption tubes of a molten nitrate salt solar absorption panel in a solar central receiver. In one preferred form, the header assembly comprises a header body and a plurality of nozzles which are manufactured from low cycle fatigue alloy 625. A plurality of flow tubes are coupled to the nozzles and in flow communication with the header body.
The use of low cycle fatigue alloy 625 as a material for the header body and nozzles allows a very thin wall header to be constructed which thermally matches the thin receiver tubes and which has a low thermal expansion coefficient. This construction thus yields very low thermal strains in the nozzle-to-header region of the header assembly. The combination of design and material used in the construction of the header assembly of the present invention results in a low cost, reliable header assembly having excellent resistance to both thermal fatigue induced cracking and chloride stress corrosion cracking without the need to utilize costly and complex thermal protection devices for the flow nozzles.